To investigate the effect of the miniband formation on the optical properties, we adopted three non-destructive methods of piezoelectric photothermal (PPT), photoluminescence (PL), and photoreflectance (PR) spectroscopies for strain-balanced InGaAs/GaAsP superlattice structure inserted GaAs p-i-n solar cells. From PR measurements, a critical energy corresponding to the subband transition of e1-hh1 was estimated for thick barrier samples, whereas two critical energies corresponding to the mini-Brillouin-zone center (Γ) and edge (Π) were obtained for 2.0-nm thin barrier sample. The PPT and PL spectra of 2.0-nm thin barrier sample also showed the different behaviors than thick barrier samples. The peak positions were located at lower photon energy side because of lowering the transition energy between Γ of e1 miniband and hh1. It was confirmed that the miniband formation causes the redshifts of the optical absorption (PPT) and radiative recombination (PL) processes.